<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>外观数列</title>
</head>
<body>
  <script>
    function fn(n) {
      let ans = '1'
      for (let i = 1; i < n; i++) {
        ans = ans.replace(/(\d)\1*/g, item => item.length + item[0])
      }
      return ans
    }
    console.log(fn(4))
  </script>
</body>
</html>